Systems for varying the rigidity of a structure responsive to external forces, such as earthquake tremors, wind or the like, to provide for the safety of the structure, are known in the prior art. One example of a variable rigidity structure under automatic control is disclosed in Japanese Patent Publication No. Sho 49-46993. Another method for controlling a building against earthquake tremors is disclosed in U.S. Pat. No. 4,799,339, which is assigned to the assignee of this patent application.
According to Japanese Patent Publication No. Sho 49-46993, external forces against the structure are detected by a detector such as a strain meter. Control elements disposed in principal positions of the structure are extended or contracted to vary the rigidity of the structure. Thus the structure is maintained in a rigid condition to cope with high wind pressures, while the structure is maintained in a resilient condition to cope with horizontal forces produced by an earthquake. However, the rigidity of the structure can only be changed between rigid and resilient conditions. Accordingly, the rigidity of the structure cannot be controlled responsive to specific vibrational characteristics of an earthquake to accommodate the many uncertain forces of deformation on the structure.
The system of U.S. Pat. No. 4,799,339 has advantages over the Japanese patent in that it provides more flexible control over the rigidity of the structure responsive to the specific vibrational characteristics of individual earthquakes. However, the disadvantage of the 4,799,339 patent system is that it requires many rigidity device installations throughout the structure and complicated control systems to deal with the many uncertain elements of force characteristic of earthquakes.